In an HVAC system having an under floor air plenum, cool or warm air is supplied to the plenum below the floor of an occupied space to be cooled or heated. In order to cool or heat the occupied space above the floor, terminals with diffuser grilles are mounted in the floor at predetermined locations to allow the cool or warm air in the plenum to flow into the occupied space above the floor. The terminals are located and oriented to accommodate the configuration of the occupied space to be cooled or heated.